Storm windows have the essential function of providing a secondary barrier against transmission of heat between the inside and outside of a window. They have generally consisted of a glass or plastic pane mounted in a frame fitting in the window opening, with a narrow space separating the storm window from the permanent window. Such storm windows, if provided with inside fastenings, can provide additional safety against unauthorized entry, along with their primary function of reducing the flow of heat.
Storm windows in general have been heavy and awkward to manage because of the necessary presence of a frame, and have not always performed their intended functions adequately because of the difficulty of providing a satisfactory seal against flow of air inward or outward past the storm window.